ABSTRACT
When talking about a revolution, the metaphor of a boiler might help people to avoid the simplistic idea that there is some particular level of discontent or tension which inevitably produces the collapse of a system; because whether a boiler will or will not explode depends not only on the pressure inside but also on the strength of the containing structure. The often adduced metaphor of the powder keg and the spark should also be borne in the mind, as it might help us to remember that the underlying conditions might build up a considerable explosive potential; and yet nothing may happen so long as no event occurs which might act as a precipitating cause. On the other hand, a much smaller potential can give rise to the many minor explosions if a lot of the sparks are flying about.
